1. Field of the Invention:
This invention relates to shell and tube heat exchangers and similar equipment and is particularly concerned with an improved arrangement for protecting the tubes adjacent the shell inlet in such equipment against erosion by the incoming shell-side fluid.
2. Description of the Prior Art:
Shell-and-tube heat exchangers are widely used for the indirect transfer of heat from one fluid to another. Typically, such an exchanger consists of an external shell having inlet and outlet ports for circulation of the shell-side fluid. An elongated bundle of tubes is positioned within the shell and provided with transverse baffles for directing the shell-side fluid back and forth across the tubes. The tubes are supported by tube sheets, one of which is normally stationary and the other of which may be either stationary or "floating" to accommodate changes in tube length due to thermal expansion. The tube bundle and shell may be arranged so that the tube-side fluid makes a single pass through the shell or instead makes two or more passes. In a single pass exchanger, the tube-side fluid is introduced into a head at one end of the shell and withdrawn from a second head at the other end. In a multiple pass unit, the exchanger will generally be provided with an external head containing one or more baffles so that the tube-side fluid can be introduced into one portion of the head and withdrawn from the other portion. An internal head within which the tube-side fluid flows from one set of tubes into another will generally be located at the other end of the tube bundle. A wide variety of different shell and tube arrangements have been employed in the past.
A major problem encountered in the design and maintenance of shell-and-tube heat exchangers is that of preventing erosion on the outside of the tubes at the point where fluid enters the heat exchanger shell. The incoming fluid, which may be a liquid, a gas, or a combination of the two, normally enters the shell at high velocity and will produce rapid erosion of the tubes adjacent the inlet if it is allowed to impinge directly on them. To avoid such erosion, it is common practice to install an impingement plate or baffle between the inlet and the first row of tubes. The use of such a plate alleviates the erosion problem but requires that sufficient space be provided for the incoming fluid to move around the plate into contact with the tubes, thus reducing the number of tubes which can be installed in a shell of given size and thereby increasing the cost of the exchanger. It also distorts the fluid flow pattern, increases the pressure drop through the exchanger, and may result in "dead" spots where little heat transfer takes place and increases corrosion may occur. An alternate procedure is to install vanes or similar members in the shell inlet to partially divert the fluid so that less direct impingement occurs but such devices are often plugged by solids entrained in the fluid and at best are only partially effective. Still another procedure is to weld protective shields onto the exposed surfaces of the outer tubes adjacent the inlet to protect them against erosion. The use of such shields has alleviated erosion at the expense of increased corrosion in the weld areas. Moreover, shields of this type cannot readily be installed on the second row of tubes in exchangers provided with triangular pitch or rotated square pitch tube bundles and hence these tubes are unprotected. As the result of these and related difficulties, none of the methods employed in the past to avoid heat exchanger tube erosion due to fluid impingement has been wholly effective.